Calling an applet from another class of the same package

[Rose Jac]

Hi,

I have an applet in class 'A'.
I have another class = B.
Now, I have created a set(...) method in A and this set method will be called by B after instantiating A and all the parameters required to build the applet wll be set.
And after that, i have called the init() method of A using the same A's object.

This is not working. A nullPointerException is present. I am not able to call the applet after setting the parameters of the applet.
Am i doing it the wrong way?

Thanks!

[Rob Spoor]

The init() method is called after the object is created. Before this time the applet has no access to anything that makes it an applet. Especially methods like getCodeBase(), getDocumentBase() and getParameter(String) are not available until init() has been called.

Can you perhaps show us the exception, and the part of code where the exception is thrown?

I'll move this thread to our Applets forum.

[Rose Jac]

The applet:

package manet;

import java.awt.*;
import java.applet.*;
import java.util.*;

class Ball
{
        int width , height;
        double x, y, xinc, yinc, r;
        Color color;
        Graphics g;
       // public Ball() {}
        public Ball( int x, int y, double xinc, double yinc, int r, Rectangle b)
        {
                this.x=x;
                this.y=y;
                this.xinc=xinc;
                this.yinc=yinc;
                this.r = r;
                width = b.width;
                height = b.height;
        }

        public void move()
        {
            x+=xinc;
            y+=yinc;

            //when the ball bumps against a boundary, it bounces off
            if(x<0 || x>width-2*r)
            {
                    xinc=-xinc;
                    x+=xinc;
            }

           if(y<0 || y>height-2*r)
            {
                   yinc=-yinc;
                   y+=yinc;
            }
        }

        public void paint(Graphics gr)
        {
                g=gr;
                g.setColor(Color.red);
                g.drawOval((int)x,(int)y,(int)(2*r),(int)(2*r));
                System.out.println("\n X is " + x + " PPP " + y);
        }
}

public class Motion extends Applet implements Runnable
{
        Thread runner;
        Image Buffer;
        Graphics gBuffer;
        Ball ball[];
        ArrayList<Ball> ball1 = new ArrayList();
        Rectangle bounds;
        static final int MAX=5;

        public void set(int x[], int y[], int r[])
        {
            for(int i = 0; i<x.length; i++)
            {
                ball1.add(new Ball(x[i], y[i], 3, 5, r[i], bounds));
            }
        }
        public void init()
        {
            setSize(1000, 600);
            bounds = getBounds();
//            bounds.width = 1000;
//            bounds.height = 700;

         //   Ball a = new Ball();
           // a.set(bounds);
            System.out.println(" bith " + bounds.width);
                Buffer=createImage(bounds.width, bounds.height);
                gBuffer=Buffer.getGraphics();

                ball=new Ball[MAX];

                int w=700;
                int h=700;

//                ball[0]=new Ball(50,20,1.5,3.0,50, bounds);
//                ball[1]=new Ball(60,100,2.0,4,100, bounds);
//                ball[2]=new Ball(15,70,13.0,5,20, bounds);
//                ball[3]=new Ball(150,60, 24.7,7.0,70, bounds);
//                ball[4]=new Ball(210,30,5.2,6.5,100, bounds);
//                ball1.add(ball[0]);
//                ball1.add(ball[1]);
//                ball1.add(ball[2]);
//                ball1.add(ball[3]);
//                ball1.add(ball[4]);
                int a = 30, b = 40, c = 2, d = 5, e = 10, i = 0;
                while(i<5)
                {
                    ball1.add(new Ball(a, b, c, d, e, bounds));
                    a = a+10;
                    b = b+10;
                    c = c+10;
                    d = d+10;
                    e = e+10;
                    i++;
                }
        }

        public void start()
        {
                if (runner == null)
                {
                        runner = new Thread (this);
                        runner.start();
                }
        }

        public void stop()
        {
                if (runner != null)
                {
                        runner.stop();
                        runner = null;
                }
        }

        public void run()
        {

                while(true)
                {
                    Calendar calendar = Calendar.getInstance();
                    long i = calendar.get(Calendar.SECOND);
                    if( i % 10 != 0)
                    {
                        System.out.println(i);
                        try {runner.sleep(15);}
                        catch (Exception e) { }

                        for(Ball e : ball1)
                                e.move();

                        repaint();
                    }
                    else
                    {
                        try {
                Thread t = Thread.currentThread();
                t.sleep(1000);
                t.resume();
            } catch (InterruptedException ex) {}
                }
            }
        }

        public void update(Graphics g)
        {
                paint(g);
        }

        public void paint(Graphics g)
        {

                gBuffer.setColor(Color.white);
                gBuffer.fillRect(0,0,bounds.width,bounds.height);
                for(Ball e: ball1)
                {
                    e.paint(gBuffer);
                }

//
//                for(int i=0;i<MAX;i++)
//                        ball[i].paint(gBuffer);

                g.drawImage (Buffer,0,0, this);

          }
}

The other class:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

package manet;

public class matt
{
    public static void main(String args[])
    {
    int x[] = new int[] { 100, 200, 300, 400};
    int y[] = new int[] { 200, 100, 300, 450};
    int r[] = new int[] { 30, 50, 100, 120};
    Motion mo = new Motion();
    mo.set(x, y, r);
    mo.init();
    }
}

Ouptut:
Exception in thread "main" java.lang.NullPointerException
at manet.Ball.<init>(Motion.java:21)
at manet.Motion.set(Motion.java:67)
at manet.matt.main(matt.java:20)
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

Do help.
Thanks

[Rob Spoor]

Obviously the reference you're using on line 21 is null. That reference is b. So we trace back, and notice that it's bounds - a variable that isn't initialized until init() is called, which happens after you call set and create the balls. However, that will still give you a NullPointerException since Component.createImage, which you are calling on line 80 will return null if the component is not displayable. Your applet isn't like this because it's not contained in a web page, not shown in appletviewer and not shown at all. But I don't see the point in this entire Image though - it will create an off-screen image that you never display.

That whole code is a horrible mess though. I'm sure you've been told before to use a javax.swing.Timer instead of a thread. You are violating the rules specified in Concurrency in Swing. You are calling deprecated methods of Thread that you really shouldn't (stop() and resume()). You are using the results of getGraphics() which you never should.

[Rose Jac]

Hi,

I am sorry for the trouble but i did read about timers. The thing is I do not have an ActionPerformed in my program.
The nth program i am trying (given below) does work fine but it is not the smooth motion i wanted to generate.

If there is a way to generate smooth motion using timers in this program, do mention.
And do note, i want the ball displayed in the frame to move every 20seconds and then stop for few seconds, say, 4 seconds. This continues till the window is closed.

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

package sample;
import java.applet.*;
import java.awt.*;
import java.util.Calendar;
import javax.swing.Timer;
import javax.swing.*;

class Ball
{
        int width , height;
        double x, y, xinc, yinc, r;
        Color color;
        Graphics g;
        public int xr = 0, yr = 0;
        public Ball() {}
        public Ball( int x, int y, double xinc, double yinc, int r, Rectangle b)
        {
                this.x=x;
                this.y=y;
                this.xinc=xinc;
                this.yinc=yinc;
                this.r = r;
                width = b.width;
                height = b.height;
        }

        public void move()
        {
            x+=xinc;
            y+=yinc;

            //when the ball bumps against a boundary, it bounces off
            if(x<0 || x>width-2*r)
            {
                    xinc=-xinc;
                    x+=xinc;
            }

           if(y<0 || y>height-2*r)
            {
                   yinc=-yinc;
                   y+=yinc;
            }
        }

        public void draw(Graphics gr)
        {
                g=gr;
                g.setColor(Color.red);
                g.drawOval((int)x,(int)y,(int)(2*r),(int)(2*r));
                System.out.println("\n X is " + x + " PPP " + y);
                xr = (int) x;
                yr = (int) y;
        }
}

public class algo1 extends JPanel
{
    Rectangle bounds;
    Ball b;
    public algo1() throws InterruptedException
    {
        JFrame frame=new JFrame("At t = 0");
        frame.add(this);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(900, 640);
        bounds = frame.getBounds();
        this.setBackground(Color.white);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        b = new Ball(100, 200, 41, 41, 50, bounds);
        while(true)
        {
           Calendar calendar = Calendar.getInstance();
           long time = calendar.get(Calendar.SECOND);
           if(time % 20 != 0)
           {
                b.move();
                repaint();
            }
            else
            {
                Thread.sleep(4000);
            }
            System.out.println("time is " + time);
        }
    }

    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        b.draw(g);
    }

    public static void main(String args[]) throws InterruptedException
    {
        algo1 a = new algo1();
    }

}

[Rose Jac]

And i want to display this window when an "OK" button is pressed in another class, which i am not able to do.
Do help.

Thanks

[Rose Jac]

So sorry again. I have used timer here, though i dont think the circle is stopping at all!
And the circle is just too fast.

import java.applet.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.util.Calendar;
import java.util.Timer;
import java.util.TimerTask;
import javax.swing.*;

class Ball
{
        int width , height;
        int x = 0, y = 0, dx = 0, dy = 0, r = 0;
        Color color;
        Graphics g;
        public Ball() {}
        public Ball( int x, int y, int xinc, int yinc, int r, Rectangle b)
        {
                this.x=x;
                this.y=y;
                dx =xinc;
                dy=yinc;
                this.r = r;
                width = b.width;
                height = b.height;
        }

        public void move()
        {

               if ((x - r + dx < 0) || (x + r + dx > width))
                    dx = -dx;
                if ((y - r + dy < 0) || (y + r + dy > height))
                    dy = -dy;
    // Move the circle.
                x += dx;
                y += dy;
        }

        public void draw(Graphics gr)
        {
                g=gr;
                g.setColor(Color.red);
                g.drawOval((int)x,(int)y,(int)(2*r),(int)(2*r));
                System.out.println(" X IS " + x);
        }
}

public class matt extends JPanel
{
    Rectangle bounds;
    Ball b;
    public matt() throws InterruptedException
    {
        JFrame frame=new JFrame("At t = 0");
        frame.add(this);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(900, 640);
        bounds = frame.getBounds();
        this.setBackground(Color.white);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        b = new Ball(150, 50, -1, 1, 50, bounds);
   

    final Timer timer= new java.util.Timer();
    TimerTask task= new TimerTask() { public void run() {

    while(true)
        {
                b.move();
                repaint();           
        }

    } };
    timer.schedule( task , 0, 5*1000);
    }
        

    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        b.draw(g);
    }

    public static void main(String args[]) throws InterruptedException
    {
        matt ma = new matt();
    }

}

Thanks.
And do tell how to open this window with the press of a button in another class.

[Darryl Burke]

1. In an earlier thread of yours, it was suggested more than once that you use javax.swing.Timer. Not java.util.Timer, which requires special treatment when used in conjunction with Swing components.

2. Why do you have a while(true) loop in the TimerTask's run() method?